Superalloys are a class of materials that have been specifically developed for high-temperature applications, such as gas turbine blades. The evolution from the 1st to 4th generation Ni-based superalloys has been motivated by the stringent demands on improved creep and fatigue resistance at elevated temperatures that is achieved by (1) increased solid solution strengthening and (2) the increased volume fraction of the precipitated γ′ phases in the solid state. In order to achieve these goals, the alloys contain increasing amounts of refractory alloying elements such as Mo, Re, Ta, and W. The as-cast microstructure in the latest generation alloys is therefore associated with increasing levels of microsegregation and is consequently required to be heat treated to dissolve the low-melting interdendritic phases and to homogenize the microstructure.
During solutioning the alloy is heated above the γ′ solvus in the γ phase-field over a period of usually about 8 hours to homogenize the γ phase. However, it has been observed that during solutioning a microstructural instability develops, particularly across regions that are scaled with NiO surface oxide, the instability being a result of incipient melting and/or a discontinuous precipitation reaction that results in a γ′ matrix with topologically close-packed (TCP) precipitates and γ-lamellae and the existence of a polycrystalline microstructure. At first sight that is unexpected, given that solutioning occurs within the γ phase-field. A cause of the instability seems to be Ni, Al, Co and Cr loss via evaporation, which destabilizes the γ phase and is followed by redistribution of refractory alloying elements.
The role of evaporation and oxidation on microstructural instability during solution heat treatment of Ni-based superalloys is described in D'Souza et al, Met. Trans. A, Vol. 44A, 2013, pp. 4764-4773.
When the surface microstructural instability occurs, extensive reworking of the component can be required. Where a turbine blade aerofoil surface is involved, such reworking can be detrimental to the shape of the aerofoil and can lead to blade non-conformance. By lowering the solutioning temperature, the surface microstructural instability can be suppressed, but this leads to under-solutioning of the bulk.
Other solutioning techniques are described in, for example, US 200510051527, JR 11-29822 and US 2004/0216813.